Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions channels of these pores are such as to allow adsorption of molecules with certain dimensions while rejecting those with larger dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves, to be used as molecular sieves in separation processes, catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
One family of crystalline microporous molecular sieves is molecular sieves containing framework tetrahedral units of silica (SiO.sub.2) and optionally alumina (AlO.sub.2). Another family of crystalline microporous molecular sieves contain framework tetrahedral units of alumina (AlO.sub.2) and phosphorous (PO.sub.2). These molecular sieves are discussed in "Introduction To Zeolite Science and Practice", (H. van Bekkum, E. M. Flanigen, J. C. Jansen ed. 1991) which is hereby incorporated by reference. Examples of such ALPO-based molecular sieves ("ABMS") include SAPO, ALPO, MeAPO, MeAPSO, ELAPO, and ELAPSO. The composition of these molecular sieves is disclosed in Table I below:
TABLE I ______________________________________ Compositional Acronyms for ALPO.sub.4 -Based Materials (Exemplary Me or Acronym Framework T-Atoms El T-Atoms) ______________________________________ AlPO Al, P SAPO Si, Al, P MeAPO Me, Al, P (Co, Fe, Mg, Mn, Zn) MeAPSO Me, Al, P, Si (Co, Fe, Mg, Mn, Zn) ElAPO El, Al, P (As, B, Be, Ga, Ge, Li, Ti) ElAPSO El, Al, P, Si (As, B, Be, Ga, Ge, Li, Ti) ______________________________________
Within a pore of the crystalline molecular sieve, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, disproportionation, alkylation, and transalkylation of aromatics are governed by constraints imposed by the channel size of the molecular sieve. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the pores to react; while product selectivity occurs when some of the products can not leave the channels or do not subsequently react. Product distributions can also be altered by transition state selectivity in which certain reactions can not occur because the reaction transition state is too large to form within the pores. Selectivity can also result from configuration constraints on diffusion where the dimensions of the molecule approach that of the pore system. Non-selective reactions on the surface of the molecular sieve, such reactions on the surface acid sites of the molecular sieve, are generally not desirable as such reactions are not subject to the shape selective constraints imposed on those reactions occurring within the channels of the molecular sieve.
ABMS have been used in the past as catalysts for hydrocarbon conversion. For instance, U.S. Pat. No. 4,741,820 involves the use in a reforming process using intermediate pore size molecular sieves such as SAPO which are bound by amorphorous material.
ABMS are usually prepared by crystallization of a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce the molecular sieve powder. Although the powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with the powder. Therefore, prior to using the powder in commercial processes, the crystals are usually bound.
The powder is typically bound by forming aggregate of the molecular sieve such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the ABMS in the presence of an amorphorous binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the ABMS be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound aggregates have much better mechanical strength than the powder, when such a bound material is used in a catalytic conversion process, the performance of the catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt.% of crystals, the binder dilutes the adsorption properties of the material. In addition, since the bound molecular sieve is prepared by extruding or otherwise forming the molecular sieve with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the molecular sieve or otherwise block access to the pores of the molecular sieve, or slow the rate of mass transfer to the pores of the molecular sieve which can reduce the effectiveness of the molecular sieve when used in hydrocarbon conversion processes and other applications. Furthermore, when the bound molecular sieve is used in catalytic conversion processes, the binder may affect the chemical reactions that are taking place within the molecular sieve and also may itself catalyze undesirable reactions which can result in the formation of undesirable products.